1. Field of the Invention
The present invention relates to a catalyst for the oxidation of ethylene to ethylene oxide consisting of a critical combination of silver, alkali metal such as cesium, and sulfur deposited on a support such as alpha alumina and to the production of ethylene oxide using the catalyst; a fluorine or chloride component optionally can be included. The catalyst is essentially free of rhenium or transition metal components.
2. Description of the Prior Art
Processes for the production of ethylene oxide involve the vapor phase oxidation of ethylene with molecular oxygen using a solid catalyst comprised of silver on a support such as alumina. There have been great efforts by many workers to improve the effectiveness and efficiency of the silver catalyst for producing ethylene oxide. U.S. Pat. No. 5,051,395 provides a comprehensive analysis of these efforts of prior workers.
Among the many prior teachings in this area is that of U.S. Pat. No. 4,007,135 (see also UK 1,491,447) which teaches variously silver catalysts for the production of ethylene and propylene oxides comprised of a promoting amount of copper, gold, magnesium, zinc, cadmium, mercury, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium, and/or preferably barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 1-15), silver catalysts for the production of propylene oxide comprising a promoting amount of at least one promoter selected from lithium, potassium, sodium, rubidium, cesium, copper, gold, magnesium, zinc, cadmium, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium and barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 16-34), as well as silver catalysts for producing ethylene oxide or propylene oxide comprising (a) a promoting amount of sodium, cesium, rubidium, and/or potassium, and (b) magnesium, strontium, calcium and/or preferably barium in a promoting amount (column 3, lines 5-8).
U.S. Pat. No. 5,057,481, and related U.S. Pat. No. 4,908,343 are concerned with silver ethylene oxide catalysts comprised of cesium and an oxyanion of a group 3b to 7b element.
U.S. Pat. No. 3,888,889 describes catalysts suitable for the oxidation of propylene to propylene oxide comprised of elemental silver modified by a compound of an element from Group 5b and 6b. Although the use of supports is mentioned, there are no examples. The use of cesium is not mentioned.
European Publication 0 266 015 deals with supported silver catalysts promoted with rhenium and a long list of possible copromoters.
U.S. Pat. No. 5,102,848 deals with catalysts suitable for the production of ethylene oxide comprising a silver impregnated support also having thereon at least one cation promoter such as cesium, and a promoter comprising (i) sulfate anion, (ii) fluoride anion, and (iii) oxyanion of an element of Group 3b to 6b inclusive of the Periodic Table. Possibly for purposes of comparison since it is outside the scope of catalyst claimed, the patent shows at columns 21 and 22 a catalyst No. 6 comprised of Ag/Cs/S/F on a support, the Cs amount being 1096 ppm.
U.S. Pat. No. 5,486,628 describes a silver catalyst promoted with alkali metal, rhenium and a rare earth or lanthanide component.
U.S. Pat. No. 5,011,807 is concerned with an ethylene oxide catalyst comprised of silver, alkali metal, a transition metal, and sulfur on alumina support. Presented for comparative purposes are catalysts comprised of silver, alkali metal and sulfur on alumina support which provide results inferior to the transition metal containing catalysts.
In the context of the bewildering and vast number of references, many of them contradictory, applicant has discovered a novel and improved catalyst for the production of ethylene oxide.
The present invention relates to an improved supported silver ethylene oxide catalyst containing a promoter combination consisting of a critical amount of both an alkali metal component, preferably cesium, together with a sulfur component, and to the catalyst preparation and use; the catalyst is essentially free of rhenium and transition metal components and optionally can contain a fluorine or chloride component.
Preferred catalysts prepared in accordance with this invention contain up to about 30% by weight of silver, expressed as metal, deposited upon the surface and throughout the pores of a porous refractory support. Silver contents higher than 20% by weight of total catalyst are effective, but result in catalysts which are unnecessarily expensive. Silver contents, expressed as metal, of about 5-20% based on weight of total catalyst are preferred, while silver contents of 8-15% are especially preferred.
In addition to silver, the catalyst of the invention also contains a promoter combination consisting of critical amounts of alkali metal and sulfur. The critical amount of alkali metal promoter component is at least 1000 ppm expressed as alkali metal based on the catalyst weight; preferably the catalyst contains 1200-6000 ppm, alkali metal based on the catalyst weight. Preferably the alkali metal is cesium although lithium, sodium, potassium, rubidium and mixtures can also be used. Impregnation procedures such as are described in U.S. Pat. No. 3,962,136 are advantageously employed for addition of the cesium component to the catalyst.
Necessary also for practice of the invention is the provision of sulfur as a promoting catalyst component in a critical amount relative to the alkali metal. The sulfur component can be added to the catalyst support impregnating solution as sulfate, eg. cesium sulfate, ammonium sulfate, p-toluene sulfonic acid, and the like. U.S. Pat. No. 4,766,105 describes the use of sulfur promoting agents, for example at column 10, lines 53-60, and this disclosure is incorporated herein by reference. The amount of sulfur (expressed as the element) based on the weight of catalyst in accordance with the invention is critical and must be 40-150% of the equivalent amount necessary to form the alkali metal sulfate, eg. Cs2 SO4, preferably 40-100% of this amount.
It has been found that the use of relatively high amounts of alkali metal, as above indicated, which normally would produce an inactive catalyst, when used in combination with the designated amounts of sulfur produces a catalyst having outstanding activity and selectivity.
The catalyst also optionally may contain a fluorine or chlorine promoter in amount expressed as the element of 10-300 ppm, preferably 30-100 ppm by weight based on the catalyst as an optional component. Ammonium fluoride or chloride, alkali metal fluoride or chloride, and the like can be used.
The catalysts are made with supports comprising alumina, silica, silica-alumina or combinations thereof. Preferred supports are those containing principally alpha-alumina, particularly those containing up to about 15 wt % silica.
Especially preferred supports have a porosity of about 0.1-1.0 cc/g and preferably about 0.2-0.7 cc/g. Preferred supports also have a relatively low surface area, i.e. about 0.2-2.0 m2/g, preferably 0.4-1.6 m2/g and most preferably 0.5-1.3 m2/g as determined by the BET method. See J. Am. Chem. Soc. 60, 3098-16 (1938). Porosities are determined by the mercury porosimeter method; see Drake and Ritter, xe2x80x9cInd. Eng. Chem. anal. Ed.,xe2x80x9d 17, 787 (1945). Pore and pore diameter distributions are determined from the surface area and apparent porosity measurements.
For use in commercial ethylene oxide production applications, the supports are desirably formed into regularly shaped pellets, spheres, rings, etc. Desirably, the support particles may have xe2x80x9cequivalent diametersxe2x80x9d in the range from 3-10 mm and preferably in the range of 4-8 mm, which are usually compatible with the internal diameter of the tubes in which the catalyst is placed. xe2x80x9cEquivalent diameterxe2x80x9d is the diameter of a sphere having the same external surface (i.e. neglecting surface within the pores of the particle) to volume ratio as the support particles being employed.
Preferably, the silver is added to the support by immersion of the support into a silver/amine impregnating solution or by the incipient wetness technique. The silver containing liquid penetrates by absorption, capillary action and/or vacuum into the pores of the support. A single impregnation or a series of impregnations, with or without intermediate drying, may be used, depending in part upon the concentration of the silver salt in the solution. To obtain catalyst having silver contents within the preferred range, suitable impregnating solutions will generally contain from 5-50 wt % silver, expressed as metal. The exact concentrations employed, of course, will depend upon, among other factors, the desired silver content, the nature of the support, the viscosity of the liquid, and solubility of the silver compound.
The impregnating solution, as already indicated, is characterized as a silver/amine solution, preferably such as is fully described in U.S. Pat. No. 3,702,259 the disclosure of which is incorporated herein by reference. The impregnation procedures described in U.S. Pat. No. 3,962,136 are advantageously employed for the cesium component.
Known prior procedures of predeposition, co-deposition and postdeposition of the various promoters can be employed.
After impregnation, any excess impregnating solution is separated and the support impregnated with silver and the promoter or promoters is calcined or activated. In the most preferred practice of the invention, calcination is carried out as described in commonly assigned U.S. Pat. No. 5,504,052 granted Apr. 2, 1996 and copending application Ser. No. 08/587,281 filed Jan. 16, 1996, the disclosures of which are incorporated herein by reference. The calcination is accomplished by heating the impregnated support, preferably at a gradual rate, to a temperature in the range 120-500xc2x0 C. for a time sufficient to convert the contained silver to silver metal and to decompose the organic materials and remove the same as volatiles.
The impregnated support is maintained under an inert atmosphere while it is above 300xc2x0 C. during the entire procedure. While not wishing to be bound by theory, it is believed that at temperatures of 300xc2x0 C. and higher oxygen is absorbed in substantial quantities into the bulk of the silver where it has an adverse effect on the catalyst characteristics. Inert atmospheres as employed in the invention are those which are essentially free of oxygen.
An alternative method of calcination is to heat the catalyst in a stream of air at a temperature not exceeding 300xc2x0 C., preferably not exceeding 280xc2x0 C.
Catalysts prepared in accordance with the invention have improved performance, especially stability, for the production of ethylene oxide by the vapor phase oxidation of ethylene with molecular oxygen. These usually involve reaction temperatures of about 150xc2x0 C. to 400xc2x0 C., usually about 200xc2x0 C. to 300xc2x0 C., and reaction pressures in the range of from 0.5 to 35 bar. Reactant feed mixtures contain 0.5 to 30% ethylene and 3 to 15% oxygen, with the balance comprising comparatively inert materials including such substances as nitrogen, carbon dioxide, methane, ethane, argon and the like. Only a portion of the ethylene usually is reacted per pass over the catalyst and after separation of the desired ethylene oxide product and the removal of appropriate purge streams and carbon dioxide to prevent uncontrolled build up of inerts and/or by-products, unreacted materials are returned to the oxidation reactor.
A disadvantage of the prior art rhenium promoted catalysts has been the instability associated with such catalysts. In accordance with the present invention, the rhenium free catalysts have advantageously high selectivity and high stability.